Your on the right track on thoughts.
First thing I'd do is as you described: Take the controller off the panel and mount it in the trailer near the battery and connect to the battery with some #12 or #10 wire. Keep it down to a few feet.
Especially with a PWM controller this works well. Been there, done some experiements with my first PWM controller.
What will result is that the battery will get the voltage at the output of the PWM contoller with almost no line loss (voltage drop). Yes, the line loss still exists, but it is "upstream" from the controller. What will happen is that as you add cable, your max output current move near Imp from near Isc. And even LONG cable runs with reasonable size cable will end up near Imp. And Imp and Isc are usually pretty close to each other on a % basis.
In a post I did a month or so ago I did comparison of current loss with cabling and PWM controller. With my setup of 160 watts, with a few feet between the panel and controller I got 9.5A into the battery. Keeping the controller at the battery and adding 100ft (yes 100ft) of #10 cable,(well actually #7 on one leg, #10 on the other) current only dropped by 0.5A down to 9.0A and charge voltage stayed basically the same. Late summer, low sun, so couldn't get enough sun for full power.
Panel current drop is because with the cable voltage IR drop, even though it may be small, you move the panel up the voltage curve in the region between Isc and Imp. So you go from something more than Imp with a very short cable to near Imp with longer cable. Once you get up the curve to Imp, current won't change much with "reasonable" additional cable voltage loss.
If you want a nice long portable cable between the panel and the controller, get a 50ft 30A RV cord when they put them on sale. Then cut the ends off. Then take the green ground wire and since it isn't needed solder it at both ends to either the black or white. (same color at both ends) Cuts the voltage drop more. The two #10s in parallel become #7AWG)
As I mentioned you should be able to run 100ft with this configuration and only suffer around 0.5A charging loss and still end up with the controller giving full output voltge.
Don't worry about the 15', get that panel in the sun!!
To connect to the controller, I put a 50A "anderson' connector on the trailer tongue and use anderson connectors on the ends of any extension cords. Makes a great low resistance quick and foolproof connector. At the panel end I put MC4 connectors on the panel wires and one end of the primary extension cord I use. Or, you could just put an anderson connector on the panel as well.
I have two 25' cords and a 50' so I can pick and choose what is the best length to keep current up.
